Conventional snow tires provide grip on icy road surfaces via stud pins fitted into the tread portion of the tire.
Typical stud pins are embedded into stud pin installation holes provided in the tread portion. When the stud pins are embedded in the stud pin installation holes, the stud pin installation holes expand in diameter. By inserting the stud pins into the stud pin installation holes in this state, the stud pins are firmly embedded in the stud pin installation holes and are thus prevented from dropping from the stud pin installation holes upon receiving breaking and driving force or lateral force from the road surface during tire rolling motion.
The stud pins each include a buried base portion and a tip portion protruding from one end surface of the buried base portion. The buried base portion is fitted into the stud pin installation hole formed in a tread surface of the tire such that the tip portion protrudes from the tread surface.
When an edge of the tip portion comes into contact with an icy road surface and then exhibits edge effect, the stud pin provides great gripping force. Thus, the edge effect has been attempted to be enhanced by increasing the edge of the tip portion that comes into contact with an icy road surface.
A stud pin is known that includes a tip portion having a tip surface shaped into a concave polygon and a side surface provided with a recessed portion to increase the edge of the tip portion (For example, see International Patent Application Publication No. WO 2014/122570).
Unfortunately, if a tire fitted with such stud pins having tip surfaces shaped into a concave polygon runs on an icy road surface, small ice chipped by the tip portions in braking may accumulate in the recessed portions of the tip portions. Such small ice accumulating in the recessed portions may decrease the edge effect of the tip portions, resulting in a decrease in braking performance. Thus, small ice accumulating in the recessed portions is required to be discharged to maintain the edge effect.